In this type of gene therapy, a patient's cancer cells are genetically modified so that they prompt the person's immune system to attack the cells, the Houston Methodist Hospital researchers explained.

"We have created a vaccine with the patient's own cancer cells, a treatment that complements, and may even enhance, what we can achieve with traditional radiation and hormonal therapies," study senior author Dr. E. Brian Butler, chair of the department of radiation oncology, said in a hospital news release.

The study included 62 patients who were divided into two groups. One group, who had cancer cells confined to the prostate, received radiation treatment. The second group, who had more aggressive prostate cancer, received both radiation and hormone treatments.

The first group received the experimental gene therapy twice, and the second group got it three times during a phase 2 clinical trial conducted between 1999 and 2003.

Two years after treatment, prostate biopsies were negative in 83 percent of the first group and 79 percent of the second group. After five years, there was no sign of cancer recurrence in 94 percent of the first group and 91 percent of the second group, the findings showed.

Five-year survival rates were 97 percent and 94 percent, respectively, which is between 5 percent and 20 percent better than radiation treatment alone, according to the study published online Dec. 12 in the Journal of Radiation Oncology.

The results are "extremely pleasing to us, considering we had patients enrolled in our protocol after other physicians deemed them incurable," lead author Dr. Bin Teh, vice chair of the department of radiation oncology, said in the news release.

"We firmly believe this will be a viable treatment strategy," Teh added.

A phase 3 clinical trial, the final evaluation of the gene therapy's safety and efficiency before it can be approved by the U.S. Food and Drug Administration, is underway, the researchers said in the news release.